1. Field of the Invention
This invention relates, in general, to the field of marine navigational devices, and in particular, to navigational apparatus which interfaces with a vessel's hyperbolic radionavigational receiver and its auto pilot.
2. Description of the Prior Art
To steer a course in order to arrive at a predetermined distant location has long been the nemesis of marine navigators. Christopher Columbus's navigational error in discovering the "new world" when, in fact, he was seeking a better route to the orient is perhaps the foremost example of such errors. Needless to say, navigational errors have caused problems which at one extreme include the loss of lives and at the other extreme include minor irritations to a recreation-seeking boater, such as missing the island of Bimini.
Beacuse of relatively recent technological advances, navigational errors of the past are no longer a problem for a properly equipped vessel. Modern day navigational aids utilizing long Range Aid to Navigator--Loran C--enable a boater to repeatedly arrive at a predetermined distant location, within a few feet thereof. Hence, navigational errors are now measured in feet rather than miles, or even hundreds of miles.
The predecessor of Loran C, Loran A, was developed during the Second World War at the Massachusetts Institute of Technology. While the initial implementation of the system was to fulfill wartime needs, it is now used by civilians as well as the military. The Loran system utilizes land-based transmitters in conjunction with onboard receivers. Each of at least two transmitters, transmits pulsed, low frequency (100 kHz) radio waves. At any given vessel position, the location of that position is a function of the distance between the vessel and each of the two transmitters. The difference in distance is in turn a function of the time difference between signals from each transmitter to the vessel. The difference in distance will lie along a hyperbolic line comprising the locus of all positions having exactly the same time difference between the times of arrival of the signals from the transmitters to the vessel. The hyperbolic line is uniformly referred to as a line of position--LOP. Since only one LOP has been established, the vessel's position has not yet been fixed. It is only known that the vessel's position is somewhere along this particular LOP.
To fully fix the vessel's position, another set of transmitters are used. In the same manner as described above, a second LOP is established. The vessel's exact position is at the intersection of the two LOPs. In practice, three, rather than four, transmitters are used. One of the transmitters being common to each set of two transmitters.
The U.S. Coast Guard is currently responsible for operation of the LORAN C system. As previously mentioned, the LORAN C is a much improved version of its predecessor, the LORAN A system. In 1974, eight LORAN C chains were in operation, which included the use of thirty-one transmitting stations in providing coverage for over sixteen million square miles. The coverage is ultimately planned to include the entire U.S. Coastal Confluence Zone and a large portion of the northern hemisphere.
For a fuller description and explanation of the LORAN C system, reference may be had to the following publications. LORAN C, User Handbook, August, 1974, C.G.--462 Department of Transportation--Coast Guard. Popular Science Magazine, August, 1980, pp 76-78; Times Mirror Magazines, Inc.; Electra Yacht Marine Electronics Handbook, 1980/1981, Electra Yacht, Inc., 6 Yacht Haven Marine Center, Stanford, Connecticut, 06902.
As mentioned, the LORAN C system is used by navigators having onboard receivers specifically designed to receive the signals transmitted at approximately 100 kHz. There are numerous, very sophisticated and fairly inexpensive receivers on today's market. The Electra Yacht publication, above, pictorially illustrates and lists the specifications and options of some of the more commonly-used receivers. In general, the receivers will visually show, by use of a light-emitting diode display, the LOPs of a particular position of the vessel. Having this information, the navigator merely refers to LOP charts to fix his position. The receivers will also convert from a LOP position to a latitude-longitude position. Perhaps one of the more important features of most if not all the receivers is the steer function. The steer function allows a navigator to steer a course along any LORAN C chart line. A steer-computed line function allows a navigator to steer along any straight line course. This latter function enables a navigator to steer along the straight line between the point of origin and the desired point of destination. Again, in general, this is accomplished by first determining the vessel's position in terms of intersecting LOPs. Then, the destination LOP's are determined from an appropriate chart. These data are then input into the receiver and the navigator turns the vessel and rudder in an approximate direction to the destination. The receiver will then visually display a vertical bar or bars of light which will be centered along the horizontal display when the vessel is exactly on course. The vertical bar will move in incremental steps to the left or the right depending upon whether the vessel is off course to the left or the right, respectively. The further the vessel is off course, the further the vertical bar will be to the left or the right on the display panel. Typically, one increment represents an off course error of 0.1 microsecond, corresponding to an error of 50 feet. Thus, if the bar of light is three increments to the right, the vessel is 150 feet off course to the right. To put the vessel back on course, it is steered to the left until such time as the bar of light is again at the center of the display console. In actual practice, steering a course by use of the LORAN C display results in the vessel making a series of large "S" curves as the vessel tracks back and forth across the computed steer line.
It should be noted that steering a computed course is totally independent of winds, currents and compass errors. In fact, a compass reading is not even needed. This is not, however, to say that a LORAN C receiver replaces a compass. It is supplemental thereto.
An auto pilot is another navigational device which has long been known to be a steering aid to ship's captains. Auto pilots, of course, automatically steer a vessel along a desired magnetic bearing. Such devices, however, are not as accurate as steering along a LORAN C computed steer line. This is because a magnetic bearing does not account for a vessel's drifting due to currents, wind, or tide. Still, for many navigational purposes, an auto pilot is a very useful device.
A very sophisticated system is obtained with a LORAN C receiver is integrated with an auto pilot. Instead of a magnetic input, however, the LORAN C computed steer line comprises the direction that the auto pilot maintains. With such a system, a vessel's captain need only be concerned with avoiding unforeseen obstacles such as other vessels, low water, uncharted islands, localized thunderstorms, and the like. The advantages of such a system are immense. Human error is virtually eliminated. Missing a point of destination is all but impossible. Indeed, if the ships of Christopher Columbus were equipped with a LORAN C receiver and an integrated auto pilot, he would have had to become famous for reasons other than for his fortuitous navigational miscalculation.
Many of today's sea-going vessels are equipped with an auto pilot and a separate LORAN C receiver. The invention herein is intended to interface with these devices and thereby provide the benefits of a combined receiver and auto pilot. The invention is intended to provide this advantage without the need to replace either the LORAN C receiver or the auto pilot. Moreover, the invention is intended to provide this advantage with a minimum of cost and simplicity of installation.
Accordingly, an object of the present invention is to provide apparatus which interfaces with existing LORAN C receivers and auto pilots to provide automatic steering in accordance with a LORAN C steer line--either a computed steer line or a charted steer line.
Another object of the present invention is to provide interfacing apparatus which allows for extremely simple connection to an existing LORAN C receiver and to an existing auto pilot.
A further object of this invention is to provide apparatus having the advantages of an integrated LORAN C receiver and auto pilot at a fraction of the cost thereof.